1. Field of the Invention
The present invention relates to an antenna element-waveguide converter used for microwave or milliwave band communication, and the radio communication device using the same.
2. Description of the Background Art
In recent years, attention is being focused on radio transmission for a high definition television broadcast (hereinafter referred to as HDTV). Since the HDTV radio transmission involves transmission of a large volume of information, a radio transmission system using milliwaves that can secure a wide transmission bandwidth is being developed. Accordingly, for application to such a radio transmission system, a compact radio communication device is being developed in which a high frequency line is converted into a waveguide, and connected to a horn antenna or the like.
FIG. 7A is an exploded perspective view of a high frequency line-waveguide converter of a conventional radio communication device described in Japanese Patent Laying-Open No. 2008-131513, and FIG. 7B shows a central longitudinal section of the high frequency line-waveguide converter of the radio communication device as assembled. The high frequency line-waveguide converter includes a coplanar line 102 provided at a surface 101a of a dielectric substrate 101, an antenna element 103 arranged in a notch area 113 of a first ground layer 111, and a waveguide 104 attached to first ground layer 111. First ground layer 111 is provided on a rear surface 101b of dielectric substrate 101, and a second ground layer 112 is provided in the middle of dielectric substrate 101. Coplanar line 102 is composed of a linear microstrip line 121 provided in surface 101a of dielectric substrate 101 and a rectangular cavity area 122. Rectangular notch area 113 is provided at a position of first ground layer 111 substantially directly below cavity area 122 of coplanar line 102.
Antenna element 103 is provided on rear surface 101b of dielectric substrate 101 so as to be located in notch area 113. As shown in FIG. 7B, this antenna element 103 is located substantially directly below a leading end 121a of microstrip line 121 of coplanar line 102. This antenna element 103 is connected to leading end 121a of microstrip line 121 with one via hole 130. As shown in FIG. 7A, antenna element 103 connected to microstrip line 121 with via hole 130 is shielded by a plurality of via holes 131 provided along the outer peripheral edge of notch area 113 and the inner peripheral edge of cavity area 122.
Waveguide 104 is a quadrangular cylindrical standard waveguide, and is attached to first ground layer 111 with opening 140 opposed to notch area 113.
More specifically, notch area 113 corresponding to the shape of cavity area 122 is set to have the same shape as opening, 140 of waveguide 104, and waveguide 104 is attached to first ground layer 111 with opening 140 aligned with this notch area 113.
The operation of the high frequency line-waveguide converter of the conventional radio communication device shown in FIGS. 7A and 7B will now be described. A microstrip line (not shown) which is an output terminal of an external apparatus (not shown) is connected to microstrip line 121 of the waveguide-high frequency line converter of this conventional example, so that a signal is input from the external apparatus. The input signal propagates through coplanar line 102 toward leading end 121 of microstrip line 121. Then, this signal passes through leading end 121a of microstrip line 121 and via hole 130 to reach antenna element 103, and is radiated from antenna element 103 to propagate through waveguide 104.
Japanese Patent Laying-Open No. 8-125432 discloses a feed horn-integrated type LNB (Low Noise Block) converter having a configuration in which many through holes are arranged to form a circular shape in an internal layer of a multilayer substrate to constitute a waveguide section, which is connected to a waveguide section of the feed horn.
However, the conventional art having the structure as described above raises the following problems.
Generally, a waveguide is a mass of metal, which is rigid and heavy, whereas a substrate is fragile and light. Therefore, how to connect these two members having different mechanical strengths has been an important structural issue for ensuring the quality of a high frequency line-waveguide converter. In this respect, in Japanese Patent Laying-Open No. 2008-131513, a substrate on which a high frequency line is arranged and a waveguide are directly attached to each other, while in Japanese Patent Laying-Open No. 8-125432, a substrate and a chassis integrally molded with a waveguide are secured with a screw and the like. However, since the thickness of the substrate and the shape of the waveguide vary within a range of dimensional tolerances depending on individual differences, merely physically pressing them one upon the other may cause insufficient contact. Insufficient contact between the substrate and the waveguide may cause a problem in that a drop in antenna gain is directly affected when, for example, an antenna such as a feed horn is integrally molded with the waveguide. If the waveguide and the substrate are pressed excessively strongly one upon the other for sufficient contact, then, a stress produced at that time may damage the substrate and components such as ICs mounted on the substrate.
To deal with these problems, the waveguide may be indirectly connected to the substrate with another material for connection between the waveguide and the substrate, such as a conducting material, interposed therebetween. However, such connection disadvantageously complicates the manufacturing process, resulting in higher product cost.